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Synergistic Extraction of Zn(II) by Mixtures of Tri-Butyl-Phosphate and Trialkyl Amine Extractant
CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 36, Issue 5, May 2008 Online English edition of the Chinese language journal
Cite this article as: Chin J Anal Chem, 2008, 36(5), 619–622.
RESEARCH PAPER
Synergistic Extraction of Zn(II) by Mixtures of Tri-Butyl-Phosphate and Trialkyl Amine Extractant JIA Qiong*, WU Jie, LI Ting-Ting, ZHOU Wei-Hong College of Chemistry, Jilin University, Changchun 130022, China
Abstract: The extraction of Zn2+ by mixtures of tri-butyl-phosphate (TBP, B) and trialkyl amine (N235, R3N, R = C8–C10) has been carried out. The mixtures have evident synergistic effects on Zn2+ with a synergistic enhancement factor of 12.34. The extraction mechanism was studied using the methods of slope analysis and constant mole. The extracted complex is determined as (R3N)1.5·ZnCl2·B. The extraction reaction is as the following:
The equilibrium constant and thermodynamic functions, ΔH, ΔG, and ΔS have been calculated, indicating that the reaction is exothermically driven. The extraction of Zn2+ and Cd2+ by the mixtures of TBP and N235 or N,N-di(1-methyl-heptyl) acetamide (N503) was also conducted. The results show that the mixtures of TBP and N235 do not have evident synergistic effects on Cd2+, while TBP-N503 mixing systems have similar effects on neither Zn2+ nor Cd2+. Moreover, the separation factors of Zn2+ and Cd2+ by TBP-N235 system was studied. It is feasible and advantageous to separate Zn2+ from bulk cadmium solutions. Key Words:
1
Tri-butyl-phosphate; Trialkyl amine; Synergistic extraction; Zinc; Cadmium
Introduction
The effect of the pollution from heavy metals on the ecosystem is one of the environmental problems that have attracted much attention. Pollution by heavy metals is usually integrated because of their similar geography, chemistry, and environmental characteristics. Zinc and cadmium are familiar heavy metal contaminations. There is approximately 0.1%–5% cadmium in zinc ores. The environmental pollution from cadmium is usually accompanied with zinc in the mining process and subsequent processes. Therefore, the separation of zinc and cadmium has drawn much attention. Liquid-liquid solvent extraction is an extensively used method for the separation of zinc and cadmium. During the past years, solvent extraction has been greatly developed and used in industry. As a branch of solvent extraction, synergistic extraction has become a common method for the separation of metal ions.
Synergistic extraction means when a sample is extracted together by two or more extractants, the extractability is higher than the sum of the extractability when extracted by a single extractant. Synergistic extraction can not only improve the extraction efficiency and improve the extraction selectivity but also enhance the stability of the extracted complexes, improve the solubility of the extracted complexes in the organic phase, eliminate emulsification and the formation of the third phase, and increase the extraction reaction rate. Synergistic extraction has attracted extensive attention in various research fields[1–4]. Since 1980s’, the extraction of zinc and cadmium by neutral organophosphorus extractant and their mixtures with other extractants has attracted much attention[3,5–10]. There are many reports about the extraction of zinc and cadmium by mixtures of neutral orgnophosphorus extractants and amine extractants. For instance, Jia et al[3] investigated the extraction effects of zinc and cadmium by mixtures of 2-ethylhexyl phosphonic acid
Received 4 September 2007; accepted 24 September 2007 * Corresponding author. Email: [email protected]; Tel: +86 431-85095622; Fax: +86 431-85095622 This work was supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (No. 2006331). Copyright © 2008, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
JIA Qiong et al. / Chinese Journal of Analytical Chemistry, 2008, 36(5): 619–622
di(2-ethylhexyl ester) (DEH/EHP) and primary amine N1923. The mixing system has evident synergistic effects on cadmium from chloride medium. However, the system has synergistic effects on zinc at low ratios of N1923 while the system has antagonistic effects at high ratios of N1923. Ma et al[9,10] studied the extraction of zinc and cadmium by mixtures of N1923 and tri-butyl-phosphate (TBP). The extracted complexes were obtained. The extraction mechanism and the factors affecting the synergistic extraction were also discussed. Trialkyl amine (N235, R3N, R = C8–C10) is an amine extractant, which is mainly used for the extraction of uranium and rare earth elements. There are few reports about the extraction of zinc and cadmium with N235[11]. In the present study, the extraction of zinc and cadmium by mixtures TBP and N235 from chloride medium was carried out. The extraction efficiency and mechanism were investigated. The extraction reaction was obtained together with equilibrium constants. The possibility of separating zinc and cadmium was also studied.
2 2.1
Experimental Apparatus and reagents
Extractants: tri-butyl-phosphate (TBP), trialkyl amine (N235, R3N, R = C8–C10), N,N-di(1-methyl-heptyl) acetamide (N503) were purchased from Shanghai Organic Reagent Company, China. All the extractants were dissolved in n-heptane and used without further purification. The stock solutions of ZnCl2, CdCl2, EDTA, and hexamethylenetetraamine were prepared with AR chemicals. ZnCl2 and CdCl2 solutions were performed at a constant ionic strength with NaCl (μ = 2.0 M). All other reagents were of analytical reagent grade. A pHS-3C digital pH meter made by Shanghai Rex Instruments Factory of China was used. 2.2
Methods
Equal volumes (5 ml each) of aqueous and organic phases were shaken at (287 ± 1) K for 30 min. After phase separation, the concentration of Zn2+ or Cd2+ in the aqueous phase was determined by titration with EDTA. The concentration of metal ions in the organic phase was determined by difference. Distribution ratios were calculated from these concentrations. Zn2+ and Cd2+ in the aqueous solutions were determined by titration with EDTA with xylenol orange as an indicator in hexamethylenetetramine buffer solutions.
3 3.1
Results and discussion Extraction of Zn2+ by mixtures of TBP and N235
3.1.1 Extraction stoichiometry of Zn2+ by mixtures of TBP and N235 The extraction of Zn2+ from chloride media by TBP, N235, and their mixtures is shown in Fig.1. XTBP represents the concentration ratio of TBP in the organic phases, whereas D the distribution ratio, i.e., the ratio between the concentration of the metal ion in the organic phase and that in the aqueous phase. It can be seen that the extraction of Zn2+ by mixtures of TBP and N235 was much higher than that by TBP or N235 alone. There was an obvious synergistic effect on the extraction of Zn2+. According to the synergistic extraction theory[12], the synergistic enhancement factor, R, can be obtained as the following equation: (1) R is calculated as 12.34. If the synergistic extraction equation is written as: (2) The subscripts “a” and “o” denote the aqueous and organic phases, respectively. The equilibrium constant K can be obtained by:
(3) Where, βi (i = 1–4) are the 1 -4 cumulative stability constants of ZnCl2[13]. If Y is expressed as: st
th
(4) The distribution ratio, D can thus be written as: (5) To examine the composition of the extracted complex and obtain the extraction reaction equation in TBP-N235 system, the distraction ratio D was determined in a series of experiments. First, D was determined at varying N235 concentrations when the aqueous solutions and TBP
Fig.1
Extraction of Zn2+ by tri-butyl-phosphate (TBP), trialkyl amine (N235) and their mixtures
[Zn2+] = 0.02 M; μ = 2.0 M; [TBP] + [N235] = 0.60 M
JIA Qiong et al. / Chinese Journal of Analytical Chemistry, 2008, 36(5): 619–622
Fig.2
Effects of equilibrium concentration of TBP and N235 on distribution ratio [Zn2+] = 0.02 M; μ = 2.0 M
concentrations are fixed. Plots of logD versus log[R3N](O) give a straight line with a slope of approximately 1.5. Then D was determined at varying TBP concentrations when the aqueous solutions and N235 concentrations were fixed. The slope of the straight line on the basis of the plots of logD versus log[B](O) is approximately 1. The results are shown in Fig.2. Therefore, the synergistic extraction can be described by: (6) The value of log K is calculated as 1.61 ± 0.05. 3.1.2 Influence of temperature on the extraction of Zn2+ by mixtures of TBP and N235 The influence of temperature on the extraction of Zn2+ by TBP-N235 was studied at fixed concentrations of TBP and N235. The plots of logD versus [1000/T (K)] shown in Fig.3 provided a straight line with a slope of 1.88. It can be seen from Fig.3 that the distribution ratio of Zn2+ increases with decreasing experimental temperatures. The change of enthalpy of the reaction, ΔH, can be determined according to the equation:
(7) ΔH is calculated as –36.09 kJ mol–1. Therefore, the change of Gibbs free energy, ΔG and the change of entropy, ΔS of the system at 14 ºC can be obtained as well. (8)
ΔG and ΔS can be calculated as –8.85 kJ mol–1 and –94.91 J K–1·mol–1, respectively. It is obvious that the sign of ΔH is negative, which indicates that the synergistic extraction of Zn2+ by TBP-N235 is an exothermic reaction, i.e., temperature is disadvantageous for the synergistic reaction, Eq.(6). The synergistic reaction was an entropy-controlled one. The sign of ΔS is also negative, which implies that the synergistic reaction tends to be more ordered. In the process of forming the extracted complex, the decrease of particles compensates the disorder caused by the hydrolyzation of the metal ions. 3.2
Extraction of Cd2+ by mixtures of TBP and N235
The extraction of Cd2+ from chloride media was carried out by TBP-N235 in heptane. The plots of D-ΧTBP were obtained when changing the ratios of TBP and N235. The results were shown in Fig.4. There were not evident synergistic effects on Cd2+ by the mixtures of TBP and N235. The different extraction effects on Zn2+ and Cd2+ by TBP-N235 might be explained from Soft-Hard-Acid-Base (HSAB) theory[14]. N235 is a hard base, whereas Zn2+ is a borderline acid and Cd2+ a soft acid. Therefore, the extraction of Zn2+ by TBP-N235 is easier than that of Cd2+, resulting in more evident synergistic effects on Zn2+ than Cd2+. 3.3 Extraction of Zn2+ and Cd2+ by mixtures of TBP and N503 N503 is a weak basic extractant and has the advantages of high stability, low water-solubility, and low volatility.
(9)
Fig.3 Effects of temperature on distribution ratio [Zn2+] = 0.02 M, μ = 2.0 M, [TBP] = [N235] = 0.30 M
Fig.4
Extraction of Cd2+ by tri-butyl-phosphate (TBP), trialkyl amine (N235) and their mixtures
[Cd2+] = 0.02 M; μ = 2.0 M; [TBP] + [N235] = 0.60 M
JIA Qiong et al. / Chinese Journal of Analytical Chemistry, 2008, 36(5): 619–622
However, there is a carbonyl joined with N atom in N503 molecule, resulting in higher steric hindrance. Therefore, the distribution ratios were lower because of the more difficult complex reaction. 3.4 Separation of Zn2+ and Cd2+ by mixtures of TBP and N235
Fig.5 Extraction of Zn2+ and Cd2+ by tri-butyl-phosphate (TBP), N,N-di(1-methyl-heptyl) acetamide (N503) and their mixtures [Zn2+] = 0.02 M; [Cd2+] = 0.02 M; μ = 2.0 M; [TBP] + [N235] = 0.60 M
In the N503 molecule, the lone pair electrons of N atom conjugate with the carbonyl atom, leading to the carbonyl part electronegative. Therefore, it is easy for N503 to form cations in strong acid solutions, which is a cheap industrial extractant with high extractability. N503 was extensively applied to the extraction of metal ions. Figure 5 shows the extraction of Zn2+ and Cd2+ by TBP-N503. The mixing system did not have evident synergistic effects on either Zn2+ or Cd2+. The differences between the distribution ratios of Zn2+ and Cd2+ were not significant, indicating that it is difficult to use TBP-N503 for the separation of Zn2+ and Cd2+. The different extraction effects of Zn2+ and Cd2+ by TBP-N235 and TBP-N503 systems might be explained from the structures of N235 and N503. Both N235 and N503 are hard bases.
Combining the results in Figs.1 and 4, the mixtures of TBP and N235 can be considered for the separation of Zn2+ and Cd2+ because of their different extractabilities. In solvent extraction, the separation factor (β) is often used to evaluate the separation abilities between two substances. If DA and DB represent the distribution ratios of A and B under the same extraction conditions, the separation factor between A and B can be defined as follows: (10) If βA/B equals to 1, A and B cannot be separated under the extraction conditions. The farther the βA/B values are from 1, the better is the separation of the two substances. The βZn/Cd values in the mixtures of TBP and N235 are shown in Table 1. It can be seen that when XTBP was in the range of 0.1–0.9, the value of βZn/Cd was from 5.80 to 8.05. Because TBP-N235 systems had a synergistic effect on Zn2+ compared with nil effect on Cd2+, the system could be used to separate Zn2+ from bulk Cd2+ solutions.
Table 1 Separation factors of Zn2+ and Cd2+ by mixtures of TBP and N235 Ratio of TBP in the organic phase ΧTBP
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Separation factor βZn/Cd
--
7.58
7.22
7.13
6.99
7.74
8.05
6.82
6.05
5.80
--
References
Desalination, 2004, 162: 169–177 [8]
[1] [2]
[4]
1459–1462
[9]
Jia Q, Zhan C H, Li D Q, Niu C J. Sep. Sci. Tech., 2004, 39:
[10] Ma G X, Li D Q. Acta Metallurgica Sinica, 1989, 25(1):
Ma G X, Li D Q. Chinese J. Inorg. Chem., 1989, 5(4): 65–71 B24–B30
Jia Q, Bi L H, Shang Q K. Ind. Eng. Chem. Res., 2003, 42:
[11] He D S, Ma M, Zhao Z H. J. Membrane Sci., 2000, 169: 53–59
4223–4227
[12] Xu G X, Wang W Q, Wu J G. At. Energy Sci. Technol., 1963, 7:
Jia Q, Li D Q, Niu C J. Solvent Extr. Ion Exch., 2002, 20: 751–764
Hangzhou University, Handbook of Analytical Chemistry,
Han S M,Li D Q. Chinese J. Appl. Chem., 1991, 8(2): 11–15
[6]
Moradkhani D, Urbani M, Cheng C Y, Askari M, Bastani D. J,
Cierpiszewski
Beijing, 1997: 155 [14] Pearson R G. J. Am. Chem. Soc., 1963, 85: 3533–3539
Hydrometallurgy, 2005, 78: 129–136 Niemczewska
487–508 [13] Analytical Chemistry Staff Room, Chemistry Department of
[5]
[7]
369–372
Jia Q, Li D Q, Niu C J. Chinese J. Anal. Chem., 2004, 32:
1111–1123 [3]
Singh D, Singh O V, Tandon S N. Anal. Chim. Acta, 1980, 115:
R,
Szymanowski
J.
Cite this article as: Chin J Anal Chem, 2008, 36(5), 619–622.
RESEARCH PAPER
Synergistic Extraction of Zn(II) by Mixtures of Tri-Butyl-Phosphate and Trialkyl Amine Extractant JIA Qiong*, WU Jie, LI Ting-Ting, ZHOU Wei-Hong College of Chemistry, Jilin University, Changchun 130022, China
Abstract: The extraction of Zn2+ by mixtures of tri-butyl-phosphate (TBP, B) and trialkyl amine (N235, R3N, R = C8–C10) has been carried out. The mixtures have evident synergistic effects on Zn2+ with a synergistic enhancement factor of 12.34. The extraction mechanism was studied using the methods of slope analysis and constant mole. The extracted complex is determined as (R3N)1.5·ZnCl2·B. The extraction reaction is as the following:
The equilibrium constant and thermodynamic functions, ΔH, ΔG, and ΔS have been calculated, indicating that the reaction is exothermically driven. The extraction of Zn2+ and Cd2+ by the mixtures of TBP and N235 or N,N-di(1-methyl-heptyl) acetamide (N503) was also conducted. The results show that the mixtures of TBP and N235 do not have evident synergistic effects on Cd2+, while TBP-N503 mixing systems have similar effects on neither Zn2+ nor Cd2+. Moreover, the separation factors of Zn2+ and Cd2+ by TBP-N235 system was studied. It is feasible and advantageous to separate Zn2+ from bulk cadmium solutions. Key Words:
1
Tri-butyl-phosphate; Trialkyl amine; Synergistic extraction; Zinc; Cadmium
Introduction
The effect of the pollution from heavy metals on the ecosystem is one of the environmental problems that have attracted much attention. Pollution by heavy metals is usually integrated because of their similar geography, chemistry, and environmental characteristics. Zinc and cadmium are familiar heavy metal contaminations. There is approximately 0.1%–5% cadmium in zinc ores. The environmental pollution from cadmium is usually accompanied with zinc in the mining process and subsequent processes. Therefore, the separation of zinc and cadmium has drawn much attention. Liquid-liquid solvent extraction is an extensively used method for the separation of zinc and cadmium. During the past years, solvent extraction has been greatly developed and used in industry. As a branch of solvent extraction, synergistic extraction has become a common method for the separation of metal ions.
Synergistic extraction means when a sample is extracted together by two or more extractants, the extractability is higher than the sum of the extractability when extracted by a single extractant. Synergistic extraction can not only improve the extraction efficiency and improve the extraction selectivity but also enhance the stability of the extracted complexes, improve the solubility of the extracted complexes in the organic phase, eliminate emulsification and the formation of the third phase, and increase the extraction reaction rate. Synergistic extraction has attracted extensive attention in various research fields[1–4]. Since 1980s’, the extraction of zinc and cadmium by neutral organophosphorus extractant and their mixtures with other extractants has attracted much attention[3,5–10]. There are many reports about the extraction of zinc and cadmium by mixtures of neutral orgnophosphorus extractants and amine extractants. For instance, Jia et al[3] investigated the extraction effects of zinc and cadmium by mixtures of 2-ethylhexyl phosphonic acid
Received 4 September 2007; accepted 24 September 2007 * Corresponding author. Email: [email protected]; Tel: +86 431-85095622; Fax: +86 431-85095622 This work was supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (No. 2006331). Copyright © 2008, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
JIA Qiong et al. / Chinese Journal of Analytical Chemistry, 2008, 36(5): 619–622
di(2-ethylhexyl ester) (DEH/EHP) and primary amine N1923. The mixing system has evident synergistic effects on cadmium from chloride medium. However, the system has synergistic effects on zinc at low ratios of N1923 while the system has antagonistic effects at high ratios of N1923. Ma et al[9,10] studied the extraction of zinc and cadmium by mixtures of N1923 and tri-butyl-phosphate (TBP). The extracted complexes were obtained. The extraction mechanism and the factors affecting the synergistic extraction were also discussed. Trialkyl amine (N235, R3N, R = C8–C10) is an amine extractant, which is mainly used for the extraction of uranium and rare earth elements. There are few reports about the extraction of zinc and cadmium with N235[11]. In the present study, the extraction of zinc and cadmium by mixtures TBP and N235 from chloride medium was carried out. The extraction efficiency and mechanism were investigated. The extraction reaction was obtained together with equilibrium constants. The possibility of separating zinc and cadmium was also studied.
2 2.1
Experimental Apparatus and reagents
Extractants: tri-butyl-phosphate (TBP), trialkyl amine (N235, R3N, R = C8–C10), N,N-di(1-methyl-heptyl) acetamide (N503) were purchased from Shanghai Organic Reagent Company, China. All the extractants were dissolved in n-heptane and used without further purification. The stock solutions of ZnCl2, CdCl2, EDTA, and hexamethylenetetraamine were prepared with AR chemicals. ZnCl2 and CdCl2 solutions were performed at a constant ionic strength with NaCl (μ = 2.0 M). All other reagents were of analytical reagent grade. A pHS-3C digital pH meter made by Shanghai Rex Instruments Factory of China was used. 2.2
Methods
Equal volumes (5 ml each) of aqueous and organic phases were shaken at (287 ± 1) K for 30 min. After phase separation, the concentration of Zn2+ or Cd2+ in the aqueous phase was determined by titration with EDTA. The concentration of metal ions in the organic phase was determined by difference. Distribution ratios were calculated from these concentrations. Zn2+ and Cd2+ in the aqueous solutions were determined by titration with EDTA with xylenol orange as an indicator in hexamethylenetetramine buffer solutions.
3 3.1
Results and discussion Extraction of Zn2+ by mixtures of TBP and N235
3.1.1 Extraction stoichiometry of Zn2+ by mixtures of TBP and N235 The extraction of Zn2+ from chloride media by TBP, N235, and their mixtures is shown in Fig.1. XTBP represents the concentration ratio of TBP in the organic phases, whereas D the distribution ratio, i.e., the ratio between the concentration of the metal ion in the organic phase and that in the aqueous phase. It can be seen that the extraction of Zn2+ by mixtures of TBP and N235 was much higher than that by TBP or N235 alone. There was an obvious synergistic effect on the extraction of Zn2+. According to the synergistic extraction theory[12], the synergistic enhancement factor, R, can be obtained as the following equation: (1) R is calculated as 12.34. If the synergistic extraction equation is written as: (2) The subscripts “a” and “o” denote the aqueous and organic phases, respectively. The equilibrium constant K can be obtained by:
(3) Where, βi (i = 1–4) are the 1 -4 cumulative stability constants of ZnCl2[13]. If Y is expressed as: st
th
(4) The distribution ratio, D can thus be written as: (5) To examine the composition of the extracted complex and obtain the extraction reaction equation in TBP-N235 system, the distraction ratio D was determined in a series of experiments. First, D was determined at varying N235 concentrations when the aqueous solutions and TBP
Fig.1
Extraction of Zn2+ by tri-butyl-phosphate (TBP), trialkyl amine (N235) and their mixtures
[Zn2+] = 0.02 M; μ = 2.0 M; [TBP] + [N235] = 0.60 M
JIA Qiong et al. / Chinese Journal of Analytical Chemistry, 2008, 36(5): 619–622
Fig.2
Effects of equilibrium concentration of TBP and N235 on distribution ratio [Zn2+] = 0.02 M; μ = 2.0 M
concentrations are fixed. Plots of logD versus log[R3N](O) give a straight line with a slope of approximately 1.5. Then D was determined at varying TBP concentrations when the aqueous solutions and N235 concentrations were fixed. The slope of the straight line on the basis of the plots of logD versus log[B](O) is approximately 1. The results are shown in Fig.2. Therefore, the synergistic extraction can be described by: (6) The value of log K is calculated as 1.61 ± 0.05. 3.1.2 Influence of temperature on the extraction of Zn2+ by mixtures of TBP and N235 The influence of temperature on the extraction of Zn2+ by TBP-N235 was studied at fixed concentrations of TBP and N235. The plots of logD versus [1000/T (K)] shown in Fig.3 provided a straight line with a slope of 1.88. It can be seen from Fig.3 that the distribution ratio of Zn2+ increases with decreasing experimental temperatures. The change of enthalpy of the reaction, ΔH, can be determined according to the equation:
(7) ΔH is calculated as –36.09 kJ mol–1. Therefore, the change of Gibbs free energy, ΔG and the change of entropy, ΔS of the system at 14 ºC can be obtained as well. (8)
ΔG and ΔS can be calculated as –8.85 kJ mol–1 and –94.91 J K–1·mol–1, respectively. It is obvious that the sign of ΔH is negative, which indicates that the synergistic extraction of Zn2+ by TBP-N235 is an exothermic reaction, i.e., temperature is disadvantageous for the synergistic reaction, Eq.(6). The synergistic reaction was an entropy-controlled one. The sign of ΔS is also negative, which implies that the synergistic reaction tends to be more ordered. In the process of forming the extracted complex, the decrease of particles compensates the disorder caused by the hydrolyzation of the metal ions. 3.2
Extraction of Cd2+ by mixtures of TBP and N235
The extraction of Cd2+ from chloride media was carried out by TBP-N235 in heptane. The plots of D-ΧTBP were obtained when changing the ratios of TBP and N235. The results were shown in Fig.4. There were not evident synergistic effects on Cd2+ by the mixtures of TBP and N235. The different extraction effects on Zn2+ and Cd2+ by TBP-N235 might be explained from Soft-Hard-Acid-Base (HSAB) theory[14]. N235 is a hard base, whereas Zn2+ is a borderline acid and Cd2+ a soft acid. Therefore, the extraction of Zn2+ by TBP-N235 is easier than that of Cd2+, resulting in more evident synergistic effects on Zn2+ than Cd2+. 3.3 Extraction of Zn2+ and Cd2+ by mixtures of TBP and N503 N503 is a weak basic extractant and has the advantages of high stability, low water-solubility, and low volatility.
(9)
Fig.3 Effects of temperature on distribution ratio [Zn2+] = 0.02 M, μ = 2.0 M, [TBP] = [N235] = 0.30 M
Fig.4
Extraction of Cd2+ by tri-butyl-phosphate (TBP), trialkyl amine (N235) and their mixtures
[Cd2+] = 0.02 M; μ = 2.0 M; [TBP] + [N235] = 0.60 M
JIA Qiong et al. / Chinese Journal of Analytical Chemistry, 2008, 36(5): 619–622
However, there is a carbonyl joined with N atom in N503 molecule, resulting in higher steric hindrance. Therefore, the distribution ratios were lower because of the more difficult complex reaction. 3.4 Separation of Zn2+ and Cd2+ by mixtures of TBP and N235
Fig.5 Extraction of Zn2+ and Cd2+ by tri-butyl-phosphate (TBP), N,N-di(1-methyl-heptyl) acetamide (N503) and their mixtures [Zn2+] = 0.02 M; [Cd2+] = 0.02 M; μ = 2.0 M; [TBP] + [N235] = 0.60 M
In the N503 molecule, the lone pair electrons of N atom conjugate with the carbonyl atom, leading to the carbonyl part electronegative. Therefore, it is easy for N503 to form cations in strong acid solutions, which is a cheap industrial extractant with high extractability. N503 was extensively applied to the extraction of metal ions. Figure 5 shows the extraction of Zn2+ and Cd2+ by TBP-N503. The mixing system did not have evident synergistic effects on either Zn2+ or Cd2+. The differences between the distribution ratios of Zn2+ and Cd2+ were not significant, indicating that it is difficult to use TBP-N503 for the separation of Zn2+ and Cd2+. The different extraction effects of Zn2+ and Cd2+ by TBP-N235 and TBP-N503 systems might be explained from the structures of N235 and N503. Both N235 and N503 are hard bases.
Combining the results in Figs.1 and 4, the mixtures of TBP and N235 can be considered for the separation of Zn2+ and Cd2+ because of their different extractabilities. In solvent extraction, the separation factor (β) is often used to evaluate the separation abilities between two substances. If DA and DB represent the distribution ratios of A and B under the same extraction conditions, the separation factor between A and B can be defined as follows: (10) If βA/B equals to 1, A and B cannot be separated under the extraction conditions. The farther the βA/B values are from 1, the better is the separation of the two substances. The βZn/Cd values in the mixtures of TBP and N235 are shown in Table 1. It can be seen that when XTBP was in the range of 0.1–0.9, the value of βZn/Cd was from 5.80 to 8.05. Because TBP-N235 systems had a synergistic effect on Zn2+ compared with nil effect on Cd2+, the system could be used to separate Zn2+ from bulk Cd2+ solutions.
Table 1 Separation factors of Zn2+ and Cd2+ by mixtures of TBP and N235 Ratio of TBP in the organic phase ΧTBP
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Separation factor βZn/Cd
--
7.58
7.22
7.13
6.99
7.74
8.05
6.82
6.05
5.80
--
References
Desalination, 2004, 162: 169–177 [8]
[1] [2]
[4]
1459–1462
[9]
Jia Q, Zhan C H, Li D Q, Niu C J. Sep. Sci. Tech., 2004, 39:
[10] Ma G X, Li D Q. Acta Metallurgica Sinica, 1989, 25(1):
Ma G X, Li D Q. Chinese J. Inorg. Chem., 1989, 5(4): 65–71 B24–B30
Jia Q, Bi L H, Shang Q K. Ind. Eng. Chem. Res., 2003, 42:
[11] He D S, Ma M, Zhao Z H. J. Membrane Sci., 2000, 169: 53–59
4223–4227
[12] Xu G X, Wang W Q, Wu J G. At. Energy Sci. Technol., 1963, 7:
Jia Q, Li D Q, Niu C J. Solvent Extr. Ion Exch., 2002, 20: 751–764
Hangzhou University, Handbook of Analytical Chemistry,
Han S M,Li D Q. Chinese J. Appl. Chem., 1991, 8(2): 11–15
[6]
Moradkhani D, Urbani M, Cheng C Y, Askari M, Bastani D. J,
Cierpiszewski
Beijing, 1997: 155 [14] Pearson R G. J. Am. Chem. Soc., 1963, 85: 3533–3539
Hydrometallurgy, 2005, 78: 129–136 Niemczewska
487–508 [13] Analytical Chemistry Staff Room, Chemistry Department of
[5]
[7]
369–372
Jia Q, Li D Q, Niu C J. Chinese J. Anal. Chem., 2004, 32:
1111–1123 [3]
Singh D, Singh O V, Tandon S N. Anal. Chim. Acta, 1980, 115:
R,
Szymanowski
J.